add clone method to checked_space_map

space_map.h

@@ -54,6 +54,7 @@ namespace persistent_data {
 	public:
 		typedef boost::shared_ptr<persistent_space_map> ptr;
 
+		// FIXME: these two should be const
 		virtual size_t root_size() = 0;
 		virtual void copy_root(void *dest, size_t len) = 0;
 	};
@@ -65,6 +66,8 @@ namespace persistent_data {
 		virtual void check(block_counter &counter) const {
 			throw std::runtime_error("not implemented");
 		}
+
+		virtual ptr clone() const = 0;
 	};
 
 	class sm_adjust {